Systems for detecting the sirens of emergency vehicles to provide a warning signal to the driver of an automobile have been known for many years. Such systems typically include a microphone for picking up sounds on the exterior of the automobile and then passing the output of that microphone through some type of frequency selective network for selecting frequencies which are characteristic of typical emergency vehicle sirens. In the simplest form, such systems employ a band-pass filter or comparable device to respond to signals only within the frequency range of emergency vehicle sirens. Whenever such a signal is received in such a system, an alarm or warning device in the form of a light or sound is operated within the automobile. Such simple systems, however, have not been satisfactory since they are easily triggered by "noise" signals having nothing to do with emergency vehicle sirens. After a number of "false alarms", a driver of an automobile equipped with such a system has a tendency to ignore the alarm when it sounds, thereby defeating the purpose of the system.
Modern automobiles are extremely well soundproofed. Most such automobiles are operated with the windows closed practically all of the time. In the winter, the heater system of the automobile is used to keep the interior warm and in the summer an air conditioning system is used to cool the interior of the vehicle. In addition, most modern automobiles have radios, or extensive sound entertainment systems in them; and if the radio is being played with the windows closed and with either the heater system or the air conditioner operating (with its accompanying fan) it is difficult, if not impossible, for the occupants of the automobile to hear the siren of an approaching emergency vehicle until such an emergency vehicle is extremely close to the automobile. In fact, in many cases, the driver and other occupants of an automobile do not hear the siren of an approaching emergency vehicle until the emergency vehicle is only a few feet away. As a result, a high potential for an accident between the automobile and the emergency vehicle exists.
To overcome some of the disadvantages of the false alarms which are possible with simple band-pass detections sytems, tone detector systems employing a plurality of narrow band tone responsive channels have been developed. Such systems then operate only when tones in all of the channels are present, or when the tones occur in a preestablished successive order.
Four patents which are directed to a type of pattern recognition for an emergency vehicle siren are the U.S. Pat. Nos. 3,735,342, to Helliker; 3,859,623, to Koehler; 4,158,190, to Stefanov; and 4,625,206, to Jensen. All of these patents are directed to systems which recognize specific frequencies within the "siren" band of signals typically produced by emergency vehicles. In addition, all of these systems attempt to minimize false triggering of the alarm circuit by noise signals.
The system disclosed in the Helliker patent provides pattern recognition by cascading the detected outputs of tuned filters with one another, so that an output signal is obtained only when the tuned filters all produce an output within a preestablished time interval. The system continuously is reset (thus establishing the preestablished time interval); so that even if noise signals should activate one or more of the tuned filter circuits, no output is obtained. It is necessary for all of the signals to occur in the proper sequence or no output is obtained. If, as a result of interference, an improperly operating siren, or an aberration in the siren frequency (such as caused by the sound bouncing off of buildings or the like), the system of Helliker may be reset without activating the alarm, even in the presence of a siren signal which it is desired to detect.
The Jensen patent is not directed to an alarm for use within an automobile, but is concerned with the actuation of a traffic signal light control to cause the signal light to be operated to cause red lights in one direction and green lights in another, as determined by the direction of the source of the siren sound pattern detected. The Jensen patent, however, is of interest for its disclosure of systems which operate to sequentially detect frequencies found in the typical range of emergency vehicle sirens. The audio tones must be detected in the proper sequence and frequency. Detection of one frequency enables the circuit to detect signals of the next higher (or lower) frequency and so on. Out-of-sequence tones disable the circuit and reset it.
The system disclosed in the Koehler patent is designed to detect a pair of spaced frequencies which are within the siren frequency range. Koehler relies upon a "wailing" siren signal which has a repetition frequency at a relatively low rate. The detected signals produce an output at the repetition frequency which then is used to trigger the alarm. Noise signals occurring at repetition rates other than the repetition frequency, even if they do include signals of the same frequencies as the two siren signal frequencies, will not have the required repetition rate and will not permit triggering of the alarm system. In an extremely noisy, heavy traffic environment, such as typically is found in large cities, it may be possible to false trigger this system.
In addition, the system of Koehler assumes a relatively uniform repetition frequency of the siren. Modern emergency vehicles, however, do not always operate the siren in the same mode. When such a vehicle approaches a traffic intersection, the driver generally switches the siren from a "wailing" operating mode to what is known as a "yelp" operating mode. The repetition frequency of a siren operating in the "yelp" mode is higher than the frequency of a "wailing" mode. As a consequence, if the Koehler system is adjusted to detect a "wailing" siren, it may not detect a "yelp" siren and vice-versa.
Another approach for tracking a "wailing" siren frequency is disclosed in the Stefanov patent. In Stefanov, however, the "wailing" frequency rate is tracked by a voltage tunable filter. If a siren is present, this "wailing" signal occurs at a fixed rate, which is generally uniform for most emergency vehicle sirens. The outputs for the voltage tunable filter controlled by the "wailing" signal then are passed through low and high frequency filters to control the operation of the alarm circuit. The voltage tunable filter provides an output corresponding to the dominate frequency; and this output is converted to a voltage by means of a frequency-to-voltage converter which then is fed back to the band-pass filter to shift the center frequency of that filter, thereby to track the dominate frequency signal. When this signal results from a "wailing" siren, the fed-back voltage from the converter constitutes a slowly and continuously varying AC signal corresponding to the pitch variation of the siren. This signal is detected by a low-pass filter and a differentiating circuit to operate an alarm within an automobile. When the dominate frequency results from noise or steady state sounds or varies at a rate other than the slowly varying AC signal, a different or random shifting of the center frequency of the band-pass filter occurs. Additional circuitry responsive to such random signal shifts generates an inhibiting signal to prevent spurious operation of the alarm within the automobile. To be effective, this system requires several cycles of operation in order to produce an alarm output. Under many conditions, the number of cycles required may be such that the alarm within the automobile is not triggered until the emergency vehicle is dangerously close. As with prior art patents which require a sequential or cascading detection of signals within the siren frequency range for proper operation, it is possible for Stefanov to fail to trigger an alarm even though an emergency vehicle siren is present, if that siren somehow is not fully operating properly in the expected manner or if sound reflections from nearby buildings and the like distort some aspects of the siren signal.
It is desirable to provide a siren detection alarm for use within an automobile which overcomes the disadvantages of the prior art, which is not subject to "false" alarms, and which accurately provides an alarm indication even in the presence of imperfect siren signals.